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Power supply for stepper motor cards This versatile power supply has been specifically designed to power our range of stepper motor controller cards. It is also handy when a fixed +5V, +12V or +18V supply is required. Design by RICK WALTERS This power supply is capable of driving several stepper motor driver cards, depending on the current consumption of the motors. It can supply around 2-2.5A with moderate amounts of ripple and both 12V and 18V DC rails are available, allowing a wide range of stepper motors to be driven. In addition, a regulated +5V supply for the logic circuitry on each card is also provided and this can readily power eight or more cards. Many of the currently available stepper motors have centre-tapped wind­ ings and are designed for op84  Silicon Chip eration from 5V. All the driver cards described in recent issues of SILICON CHIP utilise the full winding and don’t use the centre tap. For these motors the 12V supply is ideal. As you try to increase the stepping RIGHT: the transformer and PC board are mounted on an earthed metal baseplate which is secured to the bottom of the case. Note that all exposed terminals on the fuse and mains switch should be sleeved with heatshrink tubing. supply rail can be used for this purpose. Circuit description Fig.1: the mains transformer (T1) is wired with the secondaries in series and the 9V windings are full-wave rectified using diodes D1 & D2 to give the +12V (nominal) rail. Similarly, the 12V windings are full-wave rectified using D3 & D4 to give the +18V (nominal) rail. REG1 provides the +5V rail. As you can see from the circuit (Fig.1), there is not much to it. The mains transformer (T1) is wired with the secondaries in series and the 9V windings are full-wave rectified using diodes D1 & D2 to give the +12V (nominal) rail. Similarly, the 12V windings are full-wave rectified using D3 & D4 to give the +18V (nominal) rail. These two rails are filtered using separate electrolytic capacitors – 2200µF for the +12V rail and 4700µF for the +18V rail. Finally, the +12V rail is also fed to 3-terminal regulator REG1 which gives us a stable 5V supply for the logic circuits on the controller cards. Its output is filtered using a 10µF elec­ trolytic capacitor and a 0.1µF capacitor. Assembly speed of a motor, a point is reached where it stalls. The inductance of the windings prevents the current rising rapidly enough to move the armature before the next step arrives. To help overcome this, motors are often run from a higher voltage than that specified, with a series resistor in each winding to keep the current within the motor’s rating. The 18V Most of the parts are mounted on a PC board coded 10112971. Fig.2 shows the assembly details. Begin by installing nine PC stakes at all the external wiring points, then December 1997  85 Parts List 1 plastic case, 100 x 190 x 80mm 4 stick-on rubber feet 1 PC board, code 10112971, 60 x 59mm 1 front panel label, 83 x 67mm 1 power transformer, 12/9/0/9/12 VAC, DSE M2165 or equivalent 1 250VAC 2-pole mains switch, plastic body rocker type (Altonics Cat. S3212 or equiv.) 4 panel-mount banana sockets, three red, 1 black 1 cordgrip grommet 1 mains cord with moulded 3-pin plug 1 safety M205 250VAC screw-type fuseholder (Altronics S 5992 or equiv.) 1 500mA M205 fuse 1 solder lug 9 PC stakes 4 5mm-long untapped standoffs 1 3mm x 10mm long machine screw and nut 4 3mm x 15mm-long machine screws plus nuts 5 3mm star washers 4 3mm flat washers 2 4mm x 12mm-long machine screws plus nuts 2 4mm star washers 2 4mm flat washers Semiconductors 4 1N5404 power diodes (D1-D4) 1 7805 3-terminal voltage regulator (REG1) Capacitors 1 4700µF 25VW PC electrolytic 1 2200µF 16VW PC electrolytic 1 10µF 16VW PC electrolytic 1 0.1µF MKT polyester Miscellaneous 12mm-dia heatshrink tubing, 4mm-dia heatshrink tubing, medium duty hookup wire Fig.2 (left): follow this diagram when wiring up the unit. Make sure that all polarised parts are correctly oriented and take care with the mains wiring. Fig.3 (above) shows the full-size front-panel artwork. 86  Silicon Chip Fig.4: this diagram shows the dimensions and drilling details for the aluminium baseplate. install diodes D1-D4, fol­lowed by the 3-terminal regulator (REG1) and the two small ca­pacitors next to it. The large electrolytic capacitors (4700µF) can be inserted and soldered next. Be careful to observe the correct polarity here as they are likely to fail if they are put in backwards. The completed PC board is housed in standard plastic case, along with the power transformer. The front panel carries four banana sockets (0V, +5V, +12V and +18V), whole the rear panel carries the cordgrip grommet, fuse and mains switch. Both the transformer and the PC board are mounted on an aluminium baseplate (see Fig.2), which is earthed to ensure electrical safety. Drill out all the mounting holes in the base­plate, then mount the transformer and PC board in position. The transformer is secured using 4mm screws, nuts and lockwashers, while the PC board is mounted on 5mm-long standoffs and is se­cured using 3mm screws plus nuts and washers. In addition, an earth solder lug should be secured to the baseplate Make sure that all the parts on the PC board are correctly oriented. Note that PC stakes are used to terminate the external wiring connections that run from the transformer and the front panel banana socket terminals. December 1997  87 The mains switch and fuseholder are mounted on the rear panel, as shown here. Make sure that the mains cord is properly secured (see text). adjacent to one of the corner mounting holes. Be sure to use a lockwasher under the mounting nut and secure it tightly so that it cannot come loose. The front and rear panels of the case can now be drilled to accept the various hardware items. Use a small file to carefully profile the hole for the cordgrip grommet so that it is a precise fit. A slight problem here is that the plastic end panel is a bit too thick to suit the grommet. This means that you will need to chamfer the top and bottom of the hole on the inside of the panel to make sure that the grommet locks in properly (ie, the top and bottom slots in the grommet must engage the panel). We chamfered the prototype’s panel using a Stanley knife and a small file. Take your time with this job and make sure that the grommet is a neat (tight) fit. The hole for the mains switch can be Fig.5: before installing the parts, check your PC board for etching defects by comparing it with this full-size etching pattern. made by first drilling a series of small holes around the inside perimet­er of the marked area and then knocking out the centre piece and filing the hole to shape. Once again, make sure that the mains switch is a tight fit so that it’s secured properly when pushed into the mounting hole. The baseplate assembly sits directly on four standoffs moulded into the base of the case. You will have to drill 3mm holes through the centre of each standoff, so that 3mm mounting screws can be passed through from outside the case. Once this has been done, the baseplate assembly can be mounted in position and firmly secured. Now for the internal wiring. The mains cord must be secure­ly clamped by the cordgrip grommet and the Active (brown) wire connected directly to the fuseholder. The Neutral (blue) lead goes directly to switch S1, while the Earth lead (green/yellow) is soldered to the earth lug on the baseplate. Make the earth lead somewhat longer than the other two leads, so that it will be the last to come adrift if the mains cord is reefed out by brute force. The two primary leads of the power transformer go to the bottom of S1, while the remaining terminal on S1 is connected back to the second terminal on the fuseholder. Be sure to sleeve all terminals on the mains switch and fuseholder with heatshrink tubing. This is done by pushing a short length of heatshrink tubing over each lead before it is soldered. After soldering, the heatshrink is then pushed over the exposed terminal and shrunk down using a hot-air gun. Once the mains wiring has been completed, the rear panel can be slipped into position. After that, it’s simply a matter of completing the wiring to the front panel and between the PC board and the secondary terminal of the transformer. Use medium-duty hookup wire for this job. Testing Before applying power, check your wiring carefully and use a multimeter to confirm a good connection between the transformer metalwork and the earth terminal of the mains plug. This done, attach the lid, apply power and measure the voltages on the front panel sockets. You should get readings of around 18V, 12V and 5V with respect SC to the 0V terminal. 88  Silicon Chip